Fig. 23.Fig. 23.Cross between pea and rose combed fowls. (Charts of Baur and Goldschmidt.)
Fig. 23.Cross between pea and rose combed fowls. (Charts of Baur and Goldschmidt.)
A fourth case is shown in the fruit fly, where an ebony fly with long wings is mated to a grey fly with vestigial wings (fig. 24). Theoffspring are gray with long wings. If these are inbred they give 9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
Fig. 24.Fig. 24.Cross between long ebony and gray vestigial flies.
Fig. 24.Cross between long ebony and gray vestigial flies.
The possibility of interchanging characters might be illustrated over and over again. It is true not only when two pairs of characters are involved, but when three, four, or more enter the cross.
Fig. 25.Fig. 25.Diagram to show the history of the factors in the cross shown in Fig. 24.
Fig. 25.Diagram to show the history of the factors in the cross shown in Fig. 24.
It is as though we took individuals apart and put together parts of two, three or more individuals by substituting one part for another.
Not only has this power to make whatever combinations we choose great practical importance, it has even greater theoretical significance; for, it follows that the individual is not in itself the unit in heredity, but that within the germ-cells there exist smaller units concerned with the transmission of characters.
The older mystical statement of the individual as a unit in heredity has no longer any interest in the light of these discoveries, except as a past phase of biological history. We see, too, more clearly that the sorting out of factors in the germ plasm is a very different process from the influence of these factors on the development of the organism. There is today no excuse for confusing these two problems.
If mechanistic principles apply also to embryonic development then the course of development is capable of being stated as a series of chemico-physical reactions and the "individual" is merely a term to express the sum total of such reactions and should not be interpreted as something different from or more than these reactions. So long as so little is known of the actual processes involved indevelopment the use of the term "individuality", while giving the appearance of profundity, in reality often serves merely to cover ignorance and to make a mystery out of a mechanism.
The Characters of Wild Animals and Plants Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals and Plants.
Darwin based many of his conclusions concerning variation and heredity on the evidence derived from the garden and from the stock farm. Here he was handicapped to some extent, for he had at times to rely on information much of which was uncritical, and some of which was worthless.
Today we are at least better informed ontwoimportant points; one concerning thekindsof variations that furnish to the cultivator the materials for his selection; the other concerning the modes of inheritance of these variations. We know now that new characters are continually appearing in domesticated as well as in wild animals and plants, that these characters are often sharply markedoff from the original characters, and whether the differences are great or whether they are small they are transmitted alike according to Mendel's law.
Many of the characteristics of our domesticated animals and cultivated plants originated long ago, and only here and there have the records of their first appearance been preserved. In only a few instances are these records clear and definite, while the complete history of any large group of our domesticated products is unknown to us.
Within the last five or six years, however, from a common wild species of fly, the fruit fly, Drosophila ampelophila, which we have brought into the laboratory, have arisen over a hundred and twenty-five new types whose origin is completely known. Let me call attention to a few of the more interesting of these types and their modes of inheritance, comparing them with wild types in order to show that the kinds of inheritance found in domesticated races occur also in wild types. The results will show beyond dispute that the characters of wild types are inherited in preciselythe same way as are the characters of the mutant types—a fact that is not generally appreciated except by students of genetics, although it is of the most far-reaching significance for the theory of evolution.
A mutant appeared in which the eye color of the female was different from that of the male. The eye color of the mutant female is a dark eosin color, that of the male yellowish eosin. From the beginning this difference was as marked as it is to-day. Breeding experiments show that eosin eye color differs from the red color of the eye of the wild fly by a single mutant factor. Here then at a single step a type appeared that was sexually dimorphic.
Zoölogists know that sexual dimorphism is not uncommon in wild species of animals, and Darwin proposed the theory of sexual selection to account for the difference between the sexes. He assumed that the male preferred certain kinds of females differing from himself in a particular character, and thus in time through sexual selection, the sexes came to differ from each other.
Fig. 26.Fig. 26.Clover butterfly (Colias philodice) with two types of females, above; and one type of male, below.
Fig. 26.Clover butterfly (Colias philodice) with two types of females, above; and one type of male, below.
In the case of eosin eye color no such process as that postulated by Darwin to account for the differences between the sexes was involved; for the single mutation that brought about the change also brought in the dimorphism with it.
In recent years zoölogists have carefully studied several cases in which two types of female are found in the same species. In the common clover butterfly, there is a yellow and a white type of female, while the male is yellow (fig. 26). It has been shown that a single factor difference determines whether the femaleis yellow or white. The inheritance is, according to Gerould, strictly Mendelian.
Fig. 27.Fig. 27.Papilio turnus with two types of females above and one type of male below.
Fig. 27.Papilio turnus with two types of females above and one type of male below.
In Papilio turnus there exist, in the southern states, two kinds of females, one yellow like the male, one black (fig. 27). The evidence here is not so certain, but it seems probable that a single factor difference determines whether the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and India three different types of females appear,(fig. 28 to right) only one of which is like the male. Here the analysis of the breeding data shows the possibility of explaining this case as due to two pairs Mendelian factors which give in combination the three types of female.
Fig. 28.Fig. 28.Papilio polytes, with three types of female to right and one type of male above to left.
Fig. 28.Papilio polytes, with three types of female to right and one type of male above to left.
Taking these cases together, they furnish a much simpler explanation than the one proposed by Darwin. They show also that characters like these shown by wild species may follow Mendel's law.
Fig. 29.Fig. 29.Mutant race of fruit fly with intercalated duplicate mesothorax on dorsal side.
Fig. 29.Mutant race of fruit fly with intercalated duplicate mesothorax on dorsal side.
There has appeared in our cultures a fly in which the third division of the thorax with its appendages has changed into a segment like the second (fig. 29). It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference.
A factor that causes duplication in the legshas also been found. Here the interesting fact was discovered (Hoge) that duplication takes place only in the cold. At ordinary temperatures the legs are normal.
Fig. 30.Fig. 30.Mutant race of fruit fly, called eyeless; a, a' normal eye.
Fig. 30.Mutant race of fruit fly, called eyeless; a, a' normal eye.
In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost (fig. 30). This change also took place at a single step. All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye—indeed the variation is so wide that the eye may even appear like a normal eye unless carefullyexamined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk bottles, by a change in a single factor.
I may recall in this connection that wingless flies (fig. 5 f) also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings had been blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.
The preceding examples have all related to recessive characters. The next one is dominant.
Fig. 31.Fig. 31.Mutant race of fruit fly called bar to the right (normal to the left). The eye is a narrow vertical bar, the outline of the original eye is indicated.
Fig. 31.Mutant race of fruit fly called bar to the right (normal to the left). The eye is a narrow vertical bar, the outline of the original eye is indicated.
A single male appeared with a narrow vertical red bar (fig. 31) instead of the broad red oval eye. Bred to wild females the new character was found to dominate, at least to the extent that the eyes of all its offspring were narrower than the normal eye, although not so narrow as the eye of the pure stock. Around the bar there is a wide border that corresponds to the region occupied by the rest of the eye of the wild fly. It lacks however the elements of the eye. It is therefore to be looked upon as a rudimentary organ, which is, so to speak, a by-product of the dominant mutation.
The preceding cases have all involved rather great changes in some one organ of the body. The following three cases involve slight changes, and yet follow the same laws of inheritance as do the larger changes.
Fig. 32.Fig. 32.Mutant race of fruit fly, called speck. There is a minute black speck at base of wing.
Fig. 32.Mutant race of fruit fly, called speck. There is a minute black speck at base of wing.
At the base of the wings a minute black speck appeared (fig. 32). It was found to be a Mendelian character. In another case the spines on the thorax became forked or kinky (fig. 52b). This stock breeds true, and the character is inherited in strictly Mendelian fashion.
Fig. 33.Fig. 33.Mutant race of fruit fly called club. The wings often remain unexpanded and two bristles present in wild fly (b) are absent on side of thorax (c).
Fig. 33.Mutant race of fruit fly called club. The wings often remain unexpanded and two bristles present in wild fly (b) are absent on side of thorax (c).
In a certain stock a number of flies appearedin which the wing pads did not expand (fig. 33). It was found that this peculiarity is shown in only about twenty per cent of the individuals supposed to inherit it. Later it was found that this stock lacked two bristles on the sides of the thorax. By means of this knowledge the heredity of the character was easily determined. It appears that while the expansion of the wing pads fails to occur once in five times—probably because it is an environmental effect peculiar to this stock,—yet the minute difference of the presence or absence of the two lateral bristles is a constant feature of the flies that carry this particular factor.
In the preceding cases I have spoken as though a factor influenced only one part of the body. It would have been more accurate to have stated that thechiefeffect of the factor was observed in a particular part of the body. Most students of genetics realize that a factor difference usually affects more than a single character. For example, a mutant stock called rudimentary wings has as its principle characteristic very short wings (fig. 34). But the factor for rudimentary wings also produces othereffects as well. The females are almost completely sterile, while the males are fertile. The viability of the stock is poor. When flies with rudimentary wings are put into competition with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. The hind legs are also shortened. All of these effects are the results of a single factor-difference.
Fig. 34.Fig. 34.Mutant race of fruit fly, called rudimentary.
Fig. 34.Mutant race of fruit fly, called rudimentary.
One may venture the guess that some of the specific and varietal differences that arecharacteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance.
It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whosechiefinfluence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters.
I am inclined to think that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase "unit character" has done much to mislead the uninitiated as to the effects that a single change in the germ plasm may produce on the organism. Fortunately, the expression "unit character"is being less used by those students of genetics who are more careful in regard to the implications of their terminology.
There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. The grandfather has transmitted his peculiarity to half of his grandsons, but to none of his granddaughters.
Fig. 35.Fig. 35.Diagram showing a cross between a white eyed male and a red eyed female of the fruit fly. Sex linked inheritance.
Fig. 35.Diagram showing a cross between a white eyed male and a red eyed female of the fruit fly. Sex linked inheritance.
The reciprocal cross (fig. 36) is also interesting. If a white eyed female is bred to a red eyed male, all of the daughters have red eyes and all of the sons have white eyes. We call this criss-cross inheritance. If these offspring are inbred, they produce equal numbers of red eyed and white eyed females and equal numbers of red eyed and white eyed males. The ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in entire accord with Mendel's principle of segregation.
Fig. 36.Fig. 36.Diagram illustrating a cross between a red eyed male and white eyed female of the fruit fly (reciprocal cross of that shown in Fig. 35).
Fig. 36.Diagram illustrating a cross between a red eyed male and white eyed female of the fruit fly (reciprocal cross of that shown in Fig. 35).
It has been shown by Sturtevant that in a wild species of Drosophila, viz., D. repleta, two varieties of individuals exist, in one of which the thorax has large splotches and in theother type smaller splotches (fig. 37). The factors that differentiate these varieties are sex linked.
Fig. 37.Fig. 37.Two types of markings on thorax of Drosophila repleta, both found "wild". They show sex linked inheritance.
Fig. 37.Two types of markings on thorax of Drosophila repleta, both found "wild". They show sex linked inheritance.
Certain types of color blindness (fig. 38) and certain other abnormal conditions in man such as haemophilia, are transmitted as sex linked characters.
Fig. 38A.Fig. 38, A.Diagram illustrating inheritance of color blindness in man; the iris of the color-blind eye is here black.
Fig. 38, A.Diagram illustrating inheritance of color blindness in man; the iris of the color-blind eye is here black.
Fig. 38B.Fig. 38, B.Reciprocal of cross in Fig. 38 a.
Fig. 38, B.Reciprocal of cross in Fig. 38 a.
In domestic fowls sex linked inheritance has been found as the characteristic method of transmission for at least as many as six characters, but here the relation of the sexes is in a sense reversed. For instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock (fig. 39), the offspring are all barred. If these are inbred half of the daughters are black and half are barred; all of the sons are barred. The grandmother has transmitted her color to half of her granddaughters but to none of her grandsons.
Fig. 39.Fig. 39.Sex-linked inheritance in domesticated birds shown here in a cross between barred Plymouth Rock male and black Langshan female.
Fig. 39.Sex-linked inheritance in domesticated birds shown here in a cross between barred Plymouth Rock male and black Langshan female.
Fig. 40.Fig. 40.Reciprocal of Fig. 39.
In the reciprocal cross (fig. 40) black cock by barred hen, the daughters are black and the sons barred—criss-cross inheritance. These inbred give black hens and black cocks, barred hens and barred cocks.
There is a case comparable to this found in a wild species of moth, Abraxas grossulariata. A wild variation of this type is lighter in color and is known as A. lacticolor. When these two types are crossed they exhibit exactly the same type of heredity as does the black-barred combination in the domestic fowl. As shown in figure 41, lacticolor female bred to grossulariata male gives grossulariata sons and daughters. These inbred give grossulariata males and females and lacticolor females. Reciprocally lacticolor male by grossulariata female,(fig. 42) gives lacticolor daughters and grossulariata sons and these inbred give grossulariata males and females and lacticolor males and females.
Fig. 41.Fig. 41.Sex-linked inheritance in the wild moth, Abraxas grossulariata (darker) and A. lacticolor.
Fig. 41.Sex-linked inheritance in the wild moth, Abraxas grossulariata (darker) and A. lacticolor.
Fig. 42.Fig. 42.Reciprocal of Fig. 41.
Fig. 43.Fig. 43.Four wild types of Paratettix in upper line with three hybrids below.
Fig. 43.Four wild types of Paratettix in upper line with three hybrids below.
It has been found that there may be even more than two factors that show Mendelian segregation when brought together in pairs. For example, in the southern States there are several races of the grouse locust (Paratettix) that differ from each other markedly in color patterns (fig. 43). When any two individuals of these races are crossed they give, as Nabours has shown, in F2a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there are here at least nine characters, any two of which behave as a Mendelian pair. These races havearisen in nature and differ definitely and strikingly from each other, yet any two differ by only one factor difference.
Fig. 44.Fig. 44.Diagram illustrating four allelomorphs in mice, viz. gray bellied gray (wild type) (above, to left); white bellied gray (above, to right); yellow (below, to right); and black (below, to left).
Fig. 44.Diagram illustrating four allelomorphs in mice, viz. gray bellied gray (wild type) (above, to left); white bellied gray (above, to right); yellow (below, to right); and black (below, to left).
Similar relations have been found in a number of domesticated races. In mice there is a quadruple system represented by the gray house mouse, the white bellied, the yellow and the black mouse (fig. 44). In rabbits there is probably a triple system, that includes the albino, the Himalayan, and the black races. Inthe silkworm moth there have been described four types of larvae, distinguished by different color markings, that form a system of quadruple allelomorphs. In Drosophila there is a quintuple system of factors in the sex chromosome represented by eye colors, a triple system of body colors, and a triple system of factors for eye colors in the third chromosome.
Mutation and Evolution
What bearing has the appearance of these new types of Drosophila on the theory of evolution may be asked. The objection has been raised in fact that in the breeding work with Drosophila we are dealing with artificial and unnatural conditions. It has been more than implied that results obtained from the breeding pen, the seed pan, the flower pot and the milk bottle do not apply to evolution in the "open", nature "at large" or to "wild" types. To be consistent, this same objection should be extended to the use of the spectroscope in the study of the evolution of the stars, to the use of the test tube and the balance by the chemist, of the galvanometer by the physicist. All theseare unnatural instruments used to torture Nature's secrets from her. I venture to think that the real antithesis is not between unnatural and natural treatment of Nature, but rather between controlled or verifiable data on the one hand, and unrestrained generalization on the other.
If a systematist were asked whether these new races of Drosophila are comparable to wild species, he would not hesitate for a moment. He would call them all one species. If he were asked why, he would say, I think, "These races differ only in one or two striking points, while in a hundred other respects they are identical even to the minutest details." He would add, that as large a group of wild species of flies would show on the whole the reverse relations,viz., they would differ in nearly every detail and be identical in only a few points. In all this I entirely agree with the systematist, for I do not think such a group of types differing by one character each, is comparable to most wild groups of species because the difference between wild species is due to a large number of such single differences. The charactersthat have been accumulated in wild species are of significance in the maintenance of the species, or at least we are led to infer that even though the visible character that we attend to may not itself be important, one at least of the other effects of the factors that represent these characters is significant. It is, of course, hardly to be expected thatanyrandom change in as complex a mechanism as an insect would improve the mechanism, and as a matter of fact it is doubtful whether any of the mutant types so far discovered are better adapted to those conditions to which a fly of this structure and habits is already adjusted. But this is beside the mark, for modern genetics shows very positively that adaptive characters are inherited in exactly the same way as are those that are not adaptive; and I have already pointed out that we cannot study a single mutant factor without at the same time studying one of the factors responsible for normal characters, for the two together constitute the Mendelian pair.
And, finally, I want to urge on your attention a question that we are to consider in more detail in the last lecture. Evolution of wildspecies appears to have taken place by modifying and improving bit by bit the structures and habits that the animal or plant already possessed. We have seen that there are thirty mutant factors at least that have an influence on eye color, and it is probable that there are at least as many normal factors that are involved in the production of the red eye of the wild fly.
Evolution from this point of view has consisted largely in introducing new factors that influence characters already present in the animal or plant.
Such a view gives us a somewhat different picture of the process of evolution from the old idea of a ferocious struggle between the individuals of a species with the survival of the fittest and the annihilation of the less fit. Evolution assumes a more peaceful aspect. New and advantageous characters survive by incorporating themselves into the race, improving it and opening to it new opportunities. In other words, the emphasis may be placed less on the competition between the individuals of a species (because the destruction of the less fit doesnotin itselflead to anything that is new) than on the appearance of new characters and modifications of old characters that become incorporated in the species, for on these depends the evolution of the race.
THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
The discovery that Mendel made with edible peas concerning heredity has been found to apply everywhere throughout the plant and animal kingdoms—to flowering plants, to insects, snails, crustacea, fishes, amphibians, birds, and mammals (including man).
There must be something that these widely separated groups of plants and animals have in common—some simple mechanism perhaps—to give such definite and orderly series of results. There is, in fact, a mechanism, possessed alike by animals and plants, that fulfills every requirement of Mendel's principles.
The Cellular Basis of Organic Evolution and Heredity
In order to appreciate the full force of the evidence, let me first pass rapidly in review afew familiar, historical facts, that preceded the discovery of the mechanism in question.
Fig. 45.Fig. 45.Typical cell showing the cell wall, the protoplasm (with its contained materials); the nucleus with its contained chromatin and nuclear sap. (After Dahlgren.)
Fig. 45.Typical cell showing the cell wall, the protoplasm (with its contained materials); the nucleus with its contained chromatin and nuclear sap. (After Dahlgren.)
Throughout the greater part of the last century, while students of evolution and of heredity were engaged in what I may call the more general, or, shall I say, thegrosseraspects of the subject, there existed another group of students who were engaged in working out the minute structure of the material basis of the living organism. They found that organs such as the brain, the heart, the liver, the lungs, the kidneys, etc., are not themselves the units of structure, but that all these organs can be reduced to a simpler unit that repeats itself athousand-fold in every organ. We call this unit a cell (fig. 45).
The egg is a cell, and the spermatozoon is a cell. The act of fertilization is the union of two cells (fig. 47, upper figure). Simple as the process of fertilization appears to us today, its discovery swept aside a vast amount of mystical speculation concerning the rôle of the male and of the female in the act of procreation.
Within the cell a new microcosm was revealed. Every cell was found to contain a spherical body called the nucleus (fig. 46a). Within the nucleus is a network of fibres, a sap fills the interstices of the network. The network resolves itself into a definite number of threads at each division of the cell (fig. 46 b-e). These threads we call chromosomes. Each species of animals and plants possesses a characteristic number of these threads which have a definite size and sometimes a specific shape and even characteristic granules at different levels. Beyond this point our strongest microscopes fail to penetrate. Observation has reached, for the time being, its limit.
Fig. 46.Fig. 46.A series of cells in process of cell division. The chromosomes are the black threads and rods. (After Dahlgren.)
Fig. 46.A series of cells in process of cell division. The chromosomes are the black threads and rods. (After Dahlgren.)
The story is taken up at this point by a new set of students who have worked in an entirely different field. Certain observations and experiments that we have not time to considernow, led a number of biologists to conclude that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried byeachchromosome, for the number of chromosomes is limited while the number of independently inherited characters is large. In Drosophila it has been demonstrated not only that there are exactly as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate one of these groups in a particular chromosome and to state therelative positionthere of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads us finally in a mechanical, but not in a chemical sense, to the ultimate units about which the whole process of the transmission of the hereditary factors centers.
But before plunging into this somewhat technical matter (that is difficult only because it is unfamiliar), certain facts which are familiar for the most part should be recalled, because on these turns the whole of the subsequent story.
Fig. 47.Fig. 47.An egg, and the division of the egg—the so-called process of cleavage. (After Selenka.)
Fig. 47.An egg, and the division of the egg—the so-called process of cleavage. (After Selenka.)
The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg. An hour or two after fertilization the egg divides into two cells (fig. 47). Then each half divides again. Eachquarter next divides. The process continues until a large number of cells is formed and out of these organs mould themselves.
Fig. 48.Fig. 48.Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.)
Fig. 48.Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.)
At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body,whatever its function, has a common inheritance.
At an early stage in the development of the animal certain cells are set apart to form the organs of reproduction. In some animals these cells can be identified early in the cleavage (fig. 48).
The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin (fig. 49). They divide as do the other cells of the body for a long time (fig. 49, upper row). At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle (fig. 49 c).
But there comes a time when a new process appears in the germ cells (fig 49 e-h). It is essentially the same in the egg and in the sperm cells. The discovery of this process we owe to the laborious researches of many workers in many countries. The list of their names is long, and I shall not even attempt to repeat it. The chromosomes come together in pairs (fig. 49 a). Each maternal chromosome mates with a paternal chromosome of the same kind.
Fig. 49.Fig. 49.In the upper row of the diagram a typical process of nuclear division, such as takes place in the early germ cells or in the body cells. In the lower row the separation of the chromosomes that have paired. This sort of separation takes place at one of the two reduction divisions.
Fig. 49.In the upper row of the diagram a typical process of nuclear division, such as takes place in the early germ cells or in the body cells. In the lower row the separation of the chromosomes that have paired. This sort of separation takes place at one of the two reduction divisions.
Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the divisions the double chromosomes separate so that each resulting cell comes to contain some maternal andsome paternal chromosomes, i.e. one or the other member of each pair. At the other division each chromosome simply splits as in ordinary cell division.
Fig. 50.Fig. 50.The two maturation divisions of the sperm cell. Four sperms result, each with half (haploid) the full number (diploid) of chromosomes.
Fig. 50.The two maturation divisions of the sperm cell. Four sperms result, each with half (haploid) the full number (diploid) of chromosomes.
The upshot of the process is that the ripe eggs (fig. 51) and the ripe spermatozoa (fig.50) come to contain only half the total number of chromosomes.
Fig. 51.Fig. 51.The two maturation divisions of the egg. The divisions are unequal, so that two small polar bodies are formed one of these subsequently divides. The three polar bodies and the egg are comparable to the four sperms.
Fig. 51.The two maturation divisions of the egg. The divisions are unequal, so that two small polar bodies are formed one of these subsequently divides. The three polar bodies and the egg are comparable to the four sperms.
When the eggs are fertilized the whole number of chromosomes is restored again.
The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes
If the factors in heredity are carried in the chromosomes and if the chromosomes are definite structures, we should anticipate that there should be as manygroupsof characters as there are kinds of chromosomes. In only onecase has a sufficient number of characters been studied to show whether there is any correspondence between the number of hereditary groups of characters and the number of chromosomes. In the fruit fly, Drosophila ampelophila, we have found about 125 characters that are inherited in a perfectly definite way. On the opposite page is a list of some of them.
It will be observed in this list that the characters are arranged in four groups, Groups I, II, III and IV. Three of these groups are equally large or nearly so; Group IV contains only two characters. The characters are put into these groups because in heredity the members of each group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.
If the factors for these characters are carried by the chromosomes, then we should expect that those factors that are carried by the same chromosome would be inherited together, provided the chromosomes are definite structures in the cell.